Local communication networks, so-called “local area networks” (LANs), are restricted to a geographical area and consist of one or a plurality of servers and workstations, so-called nodes, which are connected to each other via a communication line network such as a coaxial cable, a glass-fibre cable or a twisted-pair cable. Various types of network topologies, such as bus, star or tree structures, are possible for LANs. LANs are operated by means of a network operating system and a network protocol.
Currently, the most widespread standard for a LAN network protocol is the Ethernet. By means of the Ethernet, data may currently be transmitted at a rate of up to 100 Mbits per second (Mbps). In the OSI layer model, the international reference model for data transmission in networks which is made up of a stack of seven layers whereby an amount of protocols is defined for each layer which allocate their services to the respective next higher level, the Ethernet is allocated to the second layer, the so-called conductor layer. In this conductor layer the data to be transmitted are bundled to form packets to which specific information for the respective communication protocol is added. Within the network, the conductor layer is responsible for transmitting the data packets from node to node as well as for error detection. Thereby, the Ethernet supports various types of communication protocols, such as the TCP/IP protocol or the IPX protocol.
In the Ethernet concept, the conductor layer is divided up into two levels, whereby the first level adds a so-called header to the data, the header comprising information required by the receiver protocol for a correct data transmission. In the second level of the Ethernet protocol, the data packets are then encapsulated by means of an additional header and a further end section, a so-called trailer, for transporting the data packets form node to node. By means of such Ethernet data packets, so-called Ethernet telegrams, data having a length of up to 1500 bytes may be transmitted.
The Ethernet furthermore determines the access method defining how the individual nodes may utilize and occupy the physical connection paths of the network. Thereby, the Ethernet operates according to the so-called carrier sense multiple access/collision detect method (CSMA/CD). In this access method, the node ready for sending checks whether the transmission path is free prior to sending. Then, the data are transmitted. As all nodes may send their data at the same time, collisions may occur. The node noticing the collision will then interrupt transmission. In order to avoid that two nodes start sending with only a small time delay, all sending nodes generate a so-called JAM signal so that all nodes located at the transmission path interrupt the processing of the currently transmitted data packet so as not to disturb transmission.
The Ethernet protocol is predominantly used for office communication networks. Due to the advantages of the Ethernet concept in the use of standard hardware and software components and due to the possibility of achieving high data transmission rates even in the case of a simple network technology, a demand exists for being able to utilize the Ethernet network communication in industrial manufacturing, as well, in order to exchange data and carry out control tasks. Currently, the Ethernet protocol, however, only allows for limited use in automation technology, in particular due to the lack of real-time functionality. For controlling machines it is necessary that a cyclic processing of the control task follows without time fluctuations, i.e. with only a small deviation from the desired cycle time in the range of a few microseconds, the reaction to the control demand occurring within a foreseeable reply time. The CSMA/CD access method used by the Ethernet, however, does not guarantee such fixed reaction times. In the case of a strong net load, the used access method may actually be responsible for the fact that Ethernet telegrams cannot be transmitted for a certain amount of time, so that a guaranteed reply time to a control demand cannot be safeguarded.
In order to still be able to achieve a certain amount of real time in Ethernet networks, such Ethernet networks are frequently configured as star topologies using a packet switching device, a so-called switch. Thereby, each node which will be referred to as participant in the following comprises a point-to-point connection to the network switch. Such a network topology consisting of point-to-point connections between participants and the central switch, however, requires complex cabling and thus involves high costs.
The switch examines each Ethernet telegram circling in the network for the address of the addressed participant and then forwards the Ethernet telegram to the addressed participant via the corresponding point-to-point connection. Thereby, switches usually operate in such a way that they gradually learn by means of the addresses which participants are located at which switch interface. By means of the thus generated dynamic address table within the switch it is decided upon receipt of an Ethernet telegram to which interface the Ethernet telegram is to be transmitted. However, if no address entry exists for the target address of the received Ethernet telegram within the switch or the received Ethernet telegram is a multicast or broadcast telegram, respectively, the received Ethernet telegram is transmitted via all interfaces.
With regard to the transmitting order, the switches usually employ the FIFO concept in which the Ethernet telegrams which were received first are also sent first. However, this prevents reliable real-time processing of the Ethernet telegrams. More current switches thus support the so-called VLAN tagging in which individual Ethernet telegrams are prioritized during transmission in order to guarantee a real-time functionality. Moreover, switches are known which transmit Ethernet telegrams at predetermined points in time due to protocol-specific identifications in order to allow for real time processing. Due to the requirement for protocol-specific identifications, however, such switches are only able to process Ethernet telegrams generated according to the respective protocol. Contrary thereto, Ethernet telegrams generated by means of other Ethernet protocols cannot be switched in a time-controlled manner in order to guarantee real-time processing.
For the known switches, however, a number of scenarios are in principle conceivable in which a real time functionality, in which the Ethernet telegram must be reliably transmitted within a predetermined time, cannot be guaranteed. The known switches always carry out an address comparison in order to forward Ethernet telegrams. If no address entry exists within the switch for the target address of the received Ethernet telegram or the received Ethernet telegram is a multicast and/or a broadcast telegram, the received Ethernet telegram is transmitted via all interfaces. The interfaces are then blocked for maximum Ethernet data length, i.e. for 150 μsec at a data transmission rate of 100 Mbits/sec, which renders real-time processing at lower cycle times, such as 100 μsec, impossible. For the time-controlled switches, the time span of the maximum Ethernet data length, i.e. 150 μsec for a data transmission rate of 100 Mbits/sec, has to be reserved in each interface for Ethernet telegrams having the predetermined protocol-specific identification. Real-time applications having smaller cycle times and using any desired Ethernet protocol may thus not be carried out in this case, either.
In order to be able to carry out real-time control tasks by means of Ethernet telegrams without the use of switches in a simple and cost-efficient manner, DE 103 04 637 proposes to connect the participants provided for the real-time application to a ring-shaped transmission path, whereby this ring-shaped transmission path is connected to a network on which Ethernet telegrams may be transmitted via a network coupler. Thereby, the network coupler is configured in such a way that an Ethernet telegram received by the network via an external interface of the network coupler is forwarded to an internal interface and outputted onto the ring-shaped transmission path, whereby, while the Ethernet telegram passes through the ring-shaped transmission path, each participant connected to the ring-shaped transmission path exchanges the user data intended for the respective participant with the Ethernet telegram which passes through the transmission path.
In this network configuration, the ring-shaped transmission path comprising the network coupler and the participants connected to the transmission path via the network coupler appear as an individual Ethernet participant with regard to the network. The network coupler and the participants connected to the transmission path share one single Ethernet contact. By processing the Ethernet telegrams on the ring-shaped transmission path while passing through the participants connected thereto, the reaction time of the individual participants is significantly reduced so that control tasks may be carried out in real time by means of the cycling Ethernet telegrams. Particularly, it is also possible to address participants requiring only a few bytes of process data in a simple and cost-efficient manner by means of standard Ethernet telegrams which usually have a data length of several 100 bytes.
However, a problem in the ring-shaped network configuration occurs if a lot of participants are connected as delays may then occur during data processing which prevent real-time processing at very short cycle times. When passing through the individual participants as well as during forwarding between the participants, time delays in the order of magnitude of several hundred nanoseconds occur which in the case of a ring-shaped connection of more than 100 participants render cycle times of 50 μsec for real-time processing difficult. In order to avoid an excess-length string, DE 103 04 637 proposes to connect several shorter strings via a conventional address-comparing switch having a control computer, whereby a filter is additionally provided in each string which enables the filtering out of Ethernet telegrams which are not intended for participants connected via the string.